Roller-hearth furnaces are well known in the art and are useful for the uniform treatment of high production items. This type of furnace is generally used for annealing, normalizing and sintering operations. Roller-hearth furnaces can be constructed as a single furnace or as a series of furnaces for zone heating and cooling.
Roller-hearth furnaces are equipped with cylindrical or disc type rollers. Disc type rollers are employed for continuously treating short lengths of sheet material to reduce the cooling effect of full contact with cylindrical type rollers and to minimize scratching by reducing the amount of contact area. When disc type rollers are employed, they are mounted on shafts in such a manner that discs on adjacent shafts are staggered. Whether disc or cylindrical rollers are employed, the shafts upon which the discs or cylindrical rolls are mounted are driven by variable speed motors either through a chain and sprocket system or through shafts and gears.
Furnaces used for heat treating operations in excess of about 1000.degree. C. are usually equipped with conveying rollers made of heat resisting nickel base alloys. Conveying rollers made of heat resisting nickel base alloys suffer from distortion and short life at the high operating temperatures particularly where frequent thermal cycling is involved, especially at furnace entrances or exits where the rollers are exposed to sharp thermal gradients. A conveying roller made of a heat resisting nickel base alloy used in a furnace operated at about 1200.degree. C. in a reducing atmosphere typically fails from cracking and distortion in about two to six months.
The life of conveying rollers made of heat resisting nickel base alloys can be further shortened when the rollers are used in non-oxidizing or reducing atmospheres. Many heat resisting nickel base alloys are designed to operate at high temperatures under oxidizing conditions. When these nickel base alloys are used at high temperatures under reducing conditions, protective oxide coatings can be destroyed, and carbides, nitrides and hydrides can be formed which can accelerate cracking and distortion, drastically lowering the life of the rollers made of these alloys.
Not only are conveying rollers made of heat resisting metal base alloys expensive but the downtime for maintenance and replacement interferes with process control and lowers overall production.